The present invention relates to a semiconductor device and the process of fabricating the same and particularly to a semiconductor device with its resistance elements constituted by, e.g., polycrystalline silicon.
It is undesirable for a semiconductor device having a high-resistance element (e.g., a resistor) formed of polycrystalline silicon, for example, a SRAM (Static type RAM) using this kind of high-resistance element as a load capacity of a memory cell, to have directly on its high-resistance element a PSG (phosphosilicate glass) film as a layer insulating film having a gettering function. This high-resistance element is formed by utilizing as the resistance an intrinsic portion of a polycrystalline silicon layer, to which diffused impurities are not added, the remainder of which polycrystalline silicon layer is constructed as a conductive layer. If the PSG film is directly formed on this resistance element (e.g., on the intrinsic portion of the polycrystalline silicon layer), phosphorus contained in the PSG film is diffused into the intrinsic portion so as to lower the value of resistance or the stability of the high-resistance element.
Accordingly, a silicon oxide (SiO.sub.2) film formed on the surface of the polycrystalline silicon high-resistance element by thermal oxidation is possible. The silicon oxide film is constructed as a shielding film for the purpose of preventing this diffusion.
However, even if such construction is applied, an impurity (phosphorus, etc.) which is introduced in other portions of, e.g., the polycrystalline silicon layer, other than the intrinsic portion, so as to make them electrically conductive, is automatically doped to the intrinsic portion during the thermal oxidation process, thus making the value of the high-resistance element unstable, because the thermal oxidation process for forming the silicon oxide film on the high-resistance element requires heating at a high temperature of about 900.degree. C.
There is also a problem of a reduced coverage of an aluminum film due to the inferior flatness of insulation films such as the, e.g., combination of thin silicon oxide film and PSG film, described above, and on which the aluminum film is formed.